Sound reproducer



Sept. 1'7, 1935. P JENSEN 2,014,621

I SOUND REPRODUCER Filed Fb. 25, 1928 lNl/E/VTGE Prune L JENSEN In:.4770 NE).

Patented Sept. 17, 1935 UNITED STATES PATENT OFFICE SOUND REPRODUCERNevada Application February 23, 1928, Serial No. 256,289

1 Claim.

My invention relates to such sound-reproducing mechanisms as use conicaldiaphragms, and is of general usefulness in such mechanisms, but it isparticularly applicable to electromagnetic reproducers of theelectrodynamic or moving coil type.

An object of my invention is to provide a diaphragm support which willallow the diaphragm great amplitude of vibration.

Another object of my invention is to provide a diaphragm constructionwhich will tend to suppress vibrations of the diaphragm other than thosein which the diaphragm moves as a unit.

Still another object of my invention is to provide a support for theperiphery of the cone which combines great flexibility with durabilityand insensitivity to atmospheric influences.

A further object is to provide means for damping out non-radiating soundwaves passing between the diaphragm and the baffle by which it issurrounded.

My invention possesses other objects and valuable features, some ofwhich will be set forth in the following description of my inventionwhich is illustrated in the drawing forming part of the specification.It is to be understood that I do not limit myself to the showing made bythe said description and drawing, as I may adopt Varying forms of myinvention within the scope of the claim.

Referring to the drawing:

Fig. 1 is a front elevation and Fig. 2 is a sectional view taken on theline 2--2 of Fig. 1 of my invention as embodied in a reproducer of themoving coil type.

Fig. 3 is a detail sectional view, on an enlarged scale, showing amodified form of diaphragm flange and supporting plate.

Fig. 4 is a view in axial section of my invention as applied to areproducer of the magnetic armature type.

Modern advances in the art of sound reproductipn have tended toward theelimination of the sound-box and horn for the radiation of acousticpower, and the advent of large diaphragms working in free air. Theselarge diaphragms may be classified as wave diaphragms which are usuallyover nine inches in diameter and obtusely conical in shape, in which thesound is radiated by elastic waves which originate in the center andtravel toward the rim; and as plunger diaphragms, usually less than nineinches in diameter, and more acutely conical than the wave diaphragms,which radiate by the motion of the diaphragm as a whole. The distinctionis somewhat arbitrary, as the wave diaphragm tends to act as a plungerfor very low notes, while the plunger type exhibits a strong tendency toradiate by wave motion in the upper register.

There are many advantages to the plunger 5 type, but in order to realizethese fully it is necessary to provide a mounting for the base of thecone having great compliance, 1. e. very small restoring force orelastance. Moreover, the plunger diaphragm must be surrounded by a 10large baflle, to prevent the compression and rarefaction waves whichoriginate simultaneously from the front and back of the diaphragm fromneutralizing each other, or, in other terms to prevent the establishmentof non-radiating 15 waves.

In the past these objects have been accomplished by securing the conicaldiaphragm to a mounting ring attached to the baffle by means of a flatflange of very flexible leather or similar 20 material. Under favorableconditions this is fairly satisfactory, but the leather is highlyhygroscopic and shrinks and swells greatly with changing humidity. Thispulls the cone out of position and out of round, causing chattering andtending 25 to the establishment of wave vibrations in extremely complexmodes. These non-homogeneous vibrations may cause excessive radiation ofsound energy of certain frequencies, with consequent distortion. 30

Broadly considered, my invention comprises the usual medium size plungerdiaphragm, preferably a cone of paper or like material. Secured to thebase of the cone is a stiff flange, which gives the cone rigidity andoffers means for attaching a positioner comprising flexible planiformmember stretched across the base of the cone. The positioner may be anysuitable material, such as paper or parchment, but I prefer to use verythin aluminum. Adjacent the center 40 of the positioner is attached arigid supporting member, which may be either a bar or a grille passingover the face of the positioner or a strut rising axially from the apexof the cone. If the positioner be sufficiently flexible and pervious to45 sound it may be a continuous imperforate sheet like a drum head, butunder ordinary circumstances it should be vented, and in my preferredform it reduces to a spider with narrow arms ex tending from thecircumference of the cone base 50 to the point of support at the center.In order to reduce the effective mass driven by the reproducer mechanismthe positioner is preferably stiffened at the center and provided with aregion of great flexibility toward the rim. This also 55 tends to makethe flexure gradual and prevents overstress of any one point which mightcause ultimate failure.

A ring for attaching the reproducer to the baffle surrounds the base ofthe cone. On this ring is a surface which is parallel to the flange onthe diaphragm, and spaced as closely to the diaphragm flange as ispossible without danger of contact. There is thus an open passage fromthe back to the front of the diaphragm within the area of the baille,but this passage is rela tively long and has a large surface incomparison with its area to provide skin friction, and it therefore actsto damp almost completely any non-radiating sound wave passing throughit, and also assists in damping wave motion in the diaphragm.

The embodiment of my invention which I have chosen for detaileddescription is in an electromagnetic sound reproducer of the moving coiltype. The driving mechanism comprises a field electromagnet which ispreferably in the form of a ferromagnetic shell 6 surrounding a coil 1.A core 8 passes thru the coil and projects thru the pole-piece 9. Anannular air gap Ii is thus formed between core and pole-piece, and whencurrent flows in the coil 1 a powerful magnetic field is set up in thegap.

The moving coil 62 lies and reacts with this field. The coil is attachedto the conical diaphragm I3 and is preferably supported and positionedby the spider M as described in detail in my co-pending application,Serial No. 229,575, filed October 29, 1927, now Patent No. 1,906,379,granted May 2, 1933.

At the base or rim of the cone is attached a stiff flange IS, theattachment beim conveniently made by notching the edge of the cone,flaring the resultant tabs, and securing the tabs to the flange withadhesive. Preferably a cylindrical flange I'I surrounds the flat flange,although. manufacturing reasons may make the use of either the flatflange or the cylindrical flange alone desirable. Such constructions areshown in 3 and 4.

Secured to the flange I5 is a flexible planiform member 2| extendingacross the base of the cone. A wide choice of materials is available forthis member, paper, parchment, hard fiber or metal, all being suitableif properly applied. I prefer to use aluminum of very thin gage, as thisgives the necessary flexibility together with permanence, insensitivityto atmospheric changes and ease of working. Its attachment to the flangeshould be of the best, and the method I have found most satisfactorycombines the use of an adhesive such as pyroxylin lacquer and rivets 22.The rivets give the necessary mechanical strength, while the adhesiveprovides an intimacy of contact which prevents buzzing due to mutualvibration of the parts.

The plate provides means for positioning and supporting the base of thecone. This is accomplished by attaching it at or near its center to astrut 23, which is fixed in the end of the projecting core 8. A hole 24is provided in the center of the positioner, slightly larger than thescrew 26 which clamps the plate between the washers 2i and against theend of the strut. The play provided around the screw permits theaccurate adjustment of the position of the cone base.

The positioner may be made of uniform stiffness in which case itsflexibility and long radius allow ample motion of the diaphragm. Iprefer, however, to stiffen the central portion as by the ribs 28, andto increase the flexibility near the rim by providing corrugations 29perpendicular to the radius. As the cone moves it then carries with itonly a small portion of the mass of the position er, the corrugationsproviding sufficient radial yield to permit the necessary amplitude ofmotion without deforming the cone even though the radius of attachmentis relatively short. This construction also tends to prevent excessiveflexure of the positioner at any one point, with consequent fatigue andfailure. It is also advisable to provide fillets 31 at the points wherethe spider is attached to the flange, to provide for distribution of thebending stress at this point.

As in the older type of instrument a mounting ring surrounds the cone.The ring 33 is attached to the field magnet by the arms 34. Lugs 36 areprovided for attaching the reproducer to its baffle 3i, and a felt orother resilient gasket 38 may be provided to seal the connection betweenbaffle and ring, and to prevent the transmission of sound waves acrossthe junction with resultant undesired sounding-board action of thebaflle.

The ring 33 is provided with a surface 39 which is parallel to theflange ii on the diaphragm and spaced as closely thereto asmanufacturing tolerances permit. The passage between the diaphra 'rn andthe bafile is thus long in comparison to its and has large surface,damping out the non-radiath1g wave thru skin friction. The air cushionin the passage also exerts a damping effect on the diaphragm, which isnegligible as far as vibration as a whole is concerned, but is highlyeffective as regards transverse or other disturbing modes of vibrationwhich impair tone quality.

A method of applying my invention to reproducers using magnetic armaturedrive is shown in Fig. i. In this case the cone 5! is driven by a pinattached at its apex, and the use of a central strut to support thepositioner 52 is impractical. In its place the ornamental grille 54,which is customarily placed over the face of the cone, may be fixed tothe ring 56 and provided with a central stud E? to which the positioneris attached. The flange 58 and corresponding surface 59 on the ring maybe as shown or may take either the form shown in Fig. 2 or that of iFig. 3 also shows another method of securing the desired flexibilitynear the outer portion of the positioner. The arm SI! of the plate isinterrupted, and a section of fabric, leather, or other highly compliantmaterial is interposed.

In the foregoing, the diaphragm has been described as a cone, owing tothe convenience and general use of this form. It is to be understood,however, that my invention may be used with any concave or even flatdiaphragm, as long as the positioner is sufliciently spaced from thesurface of the diaphragm to prevent contact and chatter.

I claim:

In a sound reproducer, a diaphragm compris" ing a cone, a stiff flangesecured to the base of said cone, a flexible spider secured to saidflange, a support attached to said spider adjacent its center, and ribson the arms of said spider adja cent its attachment to said support.

PETER. L. JENSEN.

